Method for low NOx combustion of syngas/high hydrogen fuels

ABSTRACT

The present invention provides a method for low NOx combustion of high hydrogen content fuels in gas turbines. In the method of the present invention, at least a portion of the fuel is combusted under fuel rich conditions and a portion of resulting reaction heat is transferred to combustion air prior to non-premixed combustion of the fuel.

CROSS-REFERENCE

This application claims the benefit of U.S. Provisional Application No. 60/683,719 filed May 23, 2005.

FIELD OF THE INVENTION

The present invention relates to a method for ultra-low NOx combustion of high hydrogen content fuels. In one embodiment, the present invention provides a method for lowering the adiabatic flame temperature of a fuel prior to non-premixed combustion.

BACKGROUND OF THE INVENTION

With energy usage directly related to economic growth, there has been a steady increase in the need for increased energy supplies. In the U.S., coal is abundant and comparatively low in cost. Unfortunately, conventional coal-fired steam plants, which are a major source of electrical power, are inefficient and pollute the air. Thus, there is a pressing need for cleaner, more efficient coal-fired power plants. Accordingly, Integrated Gasification Combined Cycle (“IGCC”) coal technology systems have been developed which can achieve significantly improved efficiencies in comparison to conventional steam plants. In such a system, syngas (a mixture of hydrogen and carbon monoxide) is produced by partial oxidation of coal or other carbonaceous fuel. This allows cleanup of sulfur and other impurities, including mercury, before combustion.

Concern over global warming resulting from carbon dioxide emissions from human activity, primarily the combustion of fossil fuels, has led to the need to sequester carbon. If carbon sequestration is desired, the carbon monoxide can be reacted with steam using the water gas shift reaction to form carbon dioxide and hydrogen. Carbon dioxide may then be recovered using conventional technologies known in the art. This allows pre-combustion recovery of carbon dioxide for sequestration.

As a result of the high flame speed of hydrogen, flashback is an issue with premixed dry low NOx combustion systems. Flashback remains an issue with the use of syngas as well. Regardless of whether carbon dioxide is recovered or whether air or oxygen are used for syngas production, hydrogen content of the gas typically is too high to allow use of conventional dry low NOx premixed combustion for NOx control. Therefore, diffusion flame combustion is used typically with steam or nitrogen added as a diluent to the syngas from oxygen blown gasifiers to minimize NOx. Even so, exhaust gas cleanup still may be required. Thus, such systems, though cleaner and more efficient, typically cannot achieve present standards for NOx emissions without removal of NOx.

A further problem is that the presence of diluent in the fuel increases mass flow through the turbine often requiring the bleeding off of compressor discharge air. Since bleed off of compressor air must be limited to allow sufficient air for combustion and turbine cooling, the amount of diluent which can be added to the fuel is limited. Typically, NOx cannot be reduced below about ten parts per million (“ppm”) without operational problems, including limited flame stability.

There are further efficiency loss issues. If nitrogen is added to dilute the fuel gas, there is an energy penalty related to the need to compress the nitrogen to the pressure required for mixing with the fuel gas. In addition, use of syngas in a gas turbine designed for natural gas increases turbine mass flow even without syngas dilution. Typically, to avoid excessive loads on the turbine rotor, operation is at a reduced turbine inlet temperature and/or with bleed of compressed air from the turbine compressor.

Accordingly, improved combustion systems are needed.

SUMMARY OF THE INVENTION

It has now been found that using a reactor such as that described in U.S. Pat. No. 6,394,791, the stoichiometric flame front temperature (“SFFT”) of high hydrogen content fuels can be reduced sufficiently to provide ultra-low NOx non-premixed combustion. By reacting a sufficient amount of a fuel under fuel rich conditions and transferring at least a portion of the heat of reaction to combustion air, the SFFT is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a diagrammatic representation of the combustion of a fuel in accordance with the present invention.

FIG. 2 provides a graphical representation of the overall equivalence ratio versus temperature during the combustion of a fuel in accordance with the present invention.

FIGS. 3-4 provide a graphical representation of the burner outlet temperature versus NOx in ppm.

FIGS. 5-6 provide a graphical representation of the adiabatic flame temperature versus the temperature at the wall of the reactor at various locations.

FIG. 7 provides a graphical representation of tests results obtained from the operation of a device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As shown for the example combustor 10 in FIG. 1, a twenty percent split of the combustion air 12 is mixed with the fuel 14 to form a fuel rich mixture 16 having an equivalence ratio of two, where a ratio of one is stoichiometric. Thus, a second twenty percent of the air is required to complete combustion. Complete conversion of the oxygen is assumed in a catalytic reactor 18 with sixty percent of the heat of combustion 20 (q) transferred to the balance of the combustion air 22. On contact of the reacted fuel with the remaining combustion air, only a second twenty percent of air 24 is required for stoichiometric combustion with the balance of the combustion air 22 bypassing the flame front. Thus, forty-five percent 26 of the reaction heat bypasses the flame zone reducing the heat liberated in the flame by about twenty percent.

For conventional hydrocarbon fuels, including methane, the reduction in the heat liberated in the flame is not near enough for low NOx production in modern gas turbines. As shown in FIG. 2, even with a twenty percent air split, the adiabatic flame temperature of methane in not reduced below 1600 celsius at equivalence ratios greater than 0.3. Modem industrial and utility gas turbines require primary combustion zone equivalence ratios of greater than 0.4. For such fuels, the ultra-low NOx levels possible with lean premixed combustion, such as are possible with the method of U.S. Pat. No. 6,358,040 are preferred.

An important aspect of the present invention is that the adiabatic stoichiometric flame temperature of high hydrogen content fuels can be reduced sufficiently to allow ultra low NOx diffusion flame combustion, even for the highest inlet temperature gas turbines thus allowing wide turndown. At the operating temperatures of many turbines, low NOx is achievable with air splits as low as ten or fifteen percent. With the need for carbon sequestration becoming increasing important, the art has turned to carbon-free hydrogen such as can be produced from syngas. Nitrogen dilution of the fuel may be used for NOx control. Unfortunately, a high dilution is required to reach even ten to 15 ppm NOx.

As shown in FIGS. 3 and 4, reducing the hydrogen concentration from 100 percent to 75 percent yields an unacceptably high 200 ppm NOx with conventional combustion, as demonstrated by Todd, D. M., and Battista, R. A., (2000) “Demonstrated Applicability of Hydrogen Fuel for Gas Turbines”, Proceedings of Gasification 4 the Future, Noordwijk, Netherland. The data from this reference are shown in FIGS. 2 and 3 and denoted as “Conventional GT”. Dilution to 46 percent hydrogen is required even to approach the ten ppm level; the same level that results with 75 percent hydrogen using the method of this invention employing only a ten percent air split. As shown in FIG. 4, increasing the split from ten to twenty percent increases the amount of fuel reacted and reduces NOx by greater than a factor of more than two. It should be recognized that increasing the air split for a given fuel flow increases catalytic heat release and, in turn, increases the catalyst temperature. Accordingly, more heat is transferred to the cooling combustion air stream thereby decreasing the stoichiometric flame temperature of the fuel stream and the NOx production as shown in FIG. 4.

The maximum allowable air split is determined by the allowable material temperatures. Thus, as shown in FIG. 5, the catalytic wall temperature increases as air split is increased from ten to twenty percent. FIG. 6 shows that wall temperature decreases with increase hydrogen content of nitrogen-diluted hydrogen for a given air split. This allows higher air splits for higher hydrogen content fuel.

FIG. 7 provides the results of tests performed at conditions simulating operation of the IGCC unit at Tampa Electric Polk power station using the method of this invention. Emissions results at 2550° F. adiabatic flame temperature correspond to baseload operating temperature. At this condition, NOx emissions were 0.011 lbs/MMBtu or 2.0 ppm corrected to 15% O₂. CO emissions were near zero. As reported on the web, GE report page 12 GER-4219 (May 2003) by R Jones and N. Shilling, this unit operates at less than 25 ppm NOx; however, post combustion clean-up is required for as low as 10 ppm NOx. As shown in FIG. 7, a wide turndown at low emissions is provided. In addition, very low NOx at temperatures hundreds of degrees higher than the Tampa unit combustion temperature are possible.

While the present invention has been described in considerable detail, other configurations exhibiting the characteristics taught herein for efficient and effective heat transfer mechanisms, either catalytically or non-catalytically, are contemplated. For example, other catalytic reactor designs are contemplated as well as non-catalytic gas phase combustion. Therefore, the spirit and scope of the invention should not be limited to the description of the preferred embodiments described herein. 

1. The method of achieving low NOx in operation of a gas turbine non-premixed combustion system comprising: a) obtaining a supply of fuel; b) obtaining a supply of air; c) forming a fuel rich mixture of the fuel with a first percentage of the air; d) reacting the fuel rich mixture to produce partial reaction products plus heat; e) transferring a sufficient amount of heat to a second percentage of the air; and f) combusting the partial reaction products on contact with the heated second percentage of the air.
 2. The method of claim 1 wherein the fuel comprises syngas.
 3. The method of claim 2 wherein the syngas comprises gasified coal.
 4. The method of claim 1 wherein the fuel comprises hydrogen.
 5. The method of claim 1 wherein the turbine is operated at the design turbine inlet temperature for natural gas fuels.
 6. The method of claim 1 wherein the fuel comprises a carbon reduced syngas.
 7. The method of claim 1 wherein the amount of heat transferred to the second percentage of air lowers the stoichiometric adiabatic flame temperature of the partial reaction products on contact with the heated air to a specified temperature.
 8. The method of claim 6 wherein the specified temperature is at least 200 degrees Celsius lower than that of remaining unreacted fuel.
 9. The method of either claim 2 or claim 3 wherein the amount of air in the fuel rich mixture represents at least about ten percent of the total supply of air.
 10. The method of either claim 2 or claim 3 wherein the amount of air in the fuel rich mixture represents at least about twenty percent of the total supply of air. 